This invention relates to high frequency filters and more particularly to interdigital band-pass, stripline filters having multi-parallel-coupled resonators.
High frequency interdigital band-pass filters have been used for many years by electrical circuit designers, especially by those designers of electrical circuits which are to be operated at frequencies in the microwave range. Interdigital band-pass filters have many attractive features: they are very compact; they can be fabricated of self-supporting structures which eliminate the need for dielectric supporting materials eliminating losses associated therewith; and these filters exhibit good band-pass characteristics having steep skirt responses.
However, one significant disadvantage of self-supported interdigital band-pass filters is the dependence of the bandwidth of the filter to mechanical tolerances such as the physical dimensions of the resonators and the spacing therebetween. In common circumstances, the design of the bandwidth of such filters is not realized in a fabricated filter because of the mechanical tolerances attained during the manufacture of the filter. In most cases, a new filter must be fabricated to correct for such tolerances until the correct bandwidth is obtained in conjunction with other critical filter characteristics such as, insertion loss and voltage standing wave ratios (VSWR). Several iterations of filters may be required until a suitable unit is made. Having to make a new filter, because of tolerances, may be a very expensive step in obtaining a usable filter. Moreover, these iterations can be quite time consuming which in some instances may be more of a disadvantage than the expenses involved in remachining new parts.
Thus, a need exists to enable the designer of such filters to be able to vary the bandwidth of the filter without having to build a new filter part.